Hailey-Hailey disease (HHD) is an autosomal dominant blistering skin disease, caused by mutations in ATP2C1, a Ca2+ ATPase that we have localized to the Golgi in keratinocytes. While the central role of this Ca2+ ATPase is well established in yeast, our discovery that ATP2C1 mutations caused both defects in intracellular Ca 2+sequestration and signaling, and resulted in human disease, was the first to show the importance of this Ca 2+ ATPase in mammals. Moreover, essential Ca2+ signaling proteins such as the IP3 receptor, the Ca2+ receptor, and PLC-gamma1 co-immunoprecipitate in the Golgi with the ATP2C1. Although it had been assumed that ER Ca2+ was the major intracellular Ca 2+ store involved in signaling, these recent findings suggest that ATP2C1-controlled Golgi Ca2+ is an essential component of keratinocyte Ca2+ signaling for a variety of keratinocyte functions, including adhesion and differentiation. In the first Specific Aim, we will use HHD keratinocytes to dissect how ATP2C1 controls each step along a Ca 2+ signaling pathway, starting with plasma membrane signaling components, moving next to Ca2+ store emptying and finishing with Ca 2+ influx through plasma membrane channels. In the second Specific Aim, we first will determine which pathophysiologic mechanisms underlie the abnormal actin polymerization or exacerbation of acantholysis by cytokines that we observe in HHD keratinocytes. Next, we will determine which pathophysiologic mechanisms cause decreased involucrin expression in HHD. Finally, we will assess whether reduced ATP2C1 protein in HHD results from a dominant negative mechanism. The Short-term goals of this project are: a) defining which keratinocyte signaling pathways are controlled by the ATP2C1. b) discovering pathogenic mechanisms that contribute to defective cell-to-cell adhesion or impaired differentiation in HHD keratinocytes. c) determining whether decreased ATP2C1 protein in HHD is due to a dominant negative mechanism. The Long-term goals of this project are: a) developing treatments that mitigate the acantholysis seen in HHD. b) determining whether ATP2C1 signaling is important in other Ca 2+-mediated skin diseases such as psoriasis.